Amorphous Co-Mo-Ni alloy films are prepared by electroplating. The phase transformations and microstructure evolutions during thermal annealing under hydrogen atmosphere are clarified. The structure is amorphous if annealing temperature is lower than 200 degrees C. Nanocrystallites precipitate out of the amorphous matrix if the temperature is within the range of 300 to 600 degrees C. At 600 degrees C, amorphous matrix transforms into the supersaturated epsilon-Co plus some kappa-Co3Mo-phase. Annealing at temperature higher than 800 degrees C, epsilon-Co is converted into alpha-Co, with kappa-CO3MO becoming the major phase.